The memories such as RRAM (Resistive Random Access Memory), PRAM (Phase Change Random Access Memory) and MRAM (Magnetic Random Access Memory) have a resistive device as a memory element. The high speed access and the non-volatility at power off of these devices make these promising technologies to replace existing memories.
The resistive memory devices consisting of a top electrode, a bottom electrode and the resistive memory element in between are fabricated in an array of pillar shapes on a wafer using a conventional lithography and dry etching process. A memory element MTJ (Magnetic Tunnel Junction) in an MRAM cell includes at least a pinned layer, a free layer and barrier (or junction) layer separating the pinned and free layers. The conventional patterning processes for MRAM cells includes hard mask patterning, top electrode patterning, MTJ patterning and bottom electrode patterning processes. As the device size shrinks and the cell array becomes denser, the conventional method for MTJ etching is reaching its limit in process capabilities. MTJ reactive ion etching (RIE) can result clean in sidewalls without re-deposited material across the barrier layer. However, the presence of reactive ion species during etching results in chemical reactions with MTJ layers at the sidewall of the MTJ pillar which possibly degrades and damages the magnetic properties. The presence of the damaged material may not be important when the size of the damaged area is minor compared to the non-damaged portion inside the MTJ pillar. However, the decrease of the MTJ feature size increases the total ratio of the damaged MTJ and can result in loss of the magnetic properties and increased magnetic property variation across the wafer as well.
The ion beam etching (IBE) process which is well established in the magnetic head industry could be another option for the MTJ etching because the IBE process is known to be free of chemical reactions. However, the IBE process can be applied to the small feature size MTJ with low density but not for the high density arrays with limited pitch between two MTJ cells. Eliminating re-deposition at the MTJ sidewall is one of the key concerns for the IBE process. IBE systems typically include means for mounting a wafer on a rotating stage assembly that can include several axes of rotation that control of the angle of incidence of the ion beam. One of the possible ways to remove the re-deposited material is to etch the sidewall with high incident ion beam angle. However, this IBE angle is limited by the pitch between two adjacent MTJ pillars and the height of the MTJ pillar which includes MTJ stacks and metallic hard mask. The high incident beam angle can be used with the higher pitch, low density MTJ cells but is not usable with the small pitch for the high density MTJ cells. Increased cell density will limit the incident angle and possibly leave re-deposited material at the sidewall.
The required thickness of the hard mask is dependent on the process margin for the subsequent interconnection process. A thicker hard mask will gain more margin for the subsequent interconnection process followed by MTJ etching. However, this thick hard mask will limit the IBE incident beam angle for sidewall cleaning by increasing the height of the pillar.
FIG. 1A illustrates a cross sectional view, perpendicular to the substrate surface, at a selected stage during ion beam etching (IBE) process, according to the prior art, of an MTJ MRAM pillars in an array. Note that the incident angle is measured by convention with respect to a line perpendicular to the surface of the substrate. The two ion beam incident angles are measured at the top of the upper magnetic layer and bottom of the MTJ pillar. The width of the pillars (the feature size F) is 40 nm in FIG. 1A, 30 nm in FIG. 1B, and 20 nm in FIG. 1C. The hard mask, which also serves as the top electrode, thickness is 100 nm in each of these figures. With 2 F pitch density of MTJ cells, top electrode hard mask thickness of 100 nm and at the MTJ feature size 40 nm (FIG. 1A), the incident beam angle that can reach the bottom of the pillar will be limited to 17 degrees maximum. FIG. 1C shows that the maximum incident beam angle at the bottom of the pillar decreases to 9 degrees for the 20 nm MTJ feature size. This relatively low angle IBE process will not be sufficient to remove re-deposited and/or damaged materials from the sidewall of the MTJ during etching and could result in shorting failures which will lead to the low yields.